Ink jet printers having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, ink jet printers with multiple ink jet printing heads are used, with each head being supplied with ink of a different color. These colored inks are then apolied, either alone or in combination, to the printing medium to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan, magenta, and yellow ink. In addition, black ink may be utilized for printing textual material or for producing true four-color prints.
In a common arrangement, the print medium is attached to a rotating drum, with the ink jet heads being mounted on a traveling carriage that traverses the drum axially. As the heads scan paths over the printing medium, ink drops are projected from a minute external orifice in each head to the medium so as to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
To produce images of certain colors, more than one color of ink is combined on the medium. That is, ink drops of a first color are applied to the medium and then overlayed with ink drops of a second color to produce the desired color of the image. If the drops do not converge on the same position on the medium, that is, the drops of the two colors do not overlay one another, then the color of the image is distorted. Furthermore, it is also important that drops of substantially uniform size and shape be generated by the ink jet heads. To the extent that the drops are non-uniform, the image is distorted. This distortion affects the clarity of textual images as well as of pictoral images.
In one basic type of ink jet head, ink drops are produced on demand. An exemplary drop-on-demand ink jet head is illustrated in U.S. Pat. No. 4,106,032 of Miura, et al. In the Miura ink jet head, ink is delivered to an ink chamber in the ink jet head. Whenever a drop of ink is needed, an electric pulse is applied to a piezoelectric crystal, causing the crystal to constrict. As a result, because the crystal is in intimate mechanical contact with ink in the ink chamber, a pressure wave is transmitted through the ink chamber. In response to this pressure wave, ink flows through an ink passageway in an ink chamber wall and forms an ink drop at an internal drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the drop-forming orifice outlet and through an air chamber toward a main external orifice of the ink jet head. This latter orifice is aligned with the internal orifice and leads to the printing medium. Air under pressure is delivered to the air chamber and entrains the drop of ink in a generally concentric air stream as the ink drop travels through the air chamber. This air stream increases the speed of the drops toward, and the accuracy of applying the drops to, the print medium.
In known prior art air assisted drop-on-demand ink jet heads, the outer surface of the ink chamber wall is planar and the internal ink drop-forming orifice outlet is in the plane of this outer surface. In operation, air entering the air chamber flows inwardly from all directions toward the internal ink drop-forming orifice outlet, meets at about the location of the outlet, and then turns outwardly to flow through the external ink jet head orifice and accelerates the ink drops toward the print medium.
With this construction, a zone of stagnant air exists around the internal drop-forming orifice outlet. Moreover, ink tends to flow from the ink drop-forming orifice outlet onto the outer surface of the ink chamber wall which surrounds the orifice outlet. This wetting reduces print quality by increasing the generation of one or more spurious droplets called satellites, in addition to the main droplet of interest, in response to a pressure wave from the piezoelectric crystal. In addition, if the wetting is serious enough, it is even possible that the liquid will no longer exit the internal ink drop-forming orifice outlet as drops at all. Furthermore, if the emerging ink wets the area surrounding the ink drop-forming orifice outlet asymmetrically, the ink droplet generated in response to a pressure wave is deflected in the direction of the greatest wetting. As a result, it is more difficult to address the droplets to particular positions on the print medium. A typical addressability of prior art air assisted drop-on-demand ink jet head is approximately 150 dots per inch.
Also, to compensate for the tendency of drops to deflect from straight line travel through the external ink jet head orifice toward the print medium, the ink jet heads are typically supported relatively close to the drum and supported print medium. As a result, the external orifice may become plugged with dust and debris from the print medium. In addition, in the event the print medium loosens on the drum, because of the relatively close spacing, the print medium may slap and damage the ink jet head as the drum is rotated.
In addition, Muira type air assisted drop-on-demand ink jet heads generate ink droplets, in response to a pressure pulse, of a relatively long and irregular drop train duration. The drop train duration is the time between impact of the leading edge of the first ink droplet produced by a pulse on the print medium and the impact of the trailing edge of the last ink droplet produced in response to the pulse. In addition, such prior art ink jet heads exhibit a substantial variation in the volume of ink produced in response to a pressure pulse. Furthermore, such prior art ink jet heads are subject to the problem of ingestion of air bubbles into the ink drop-forming orifice outlet. Such bubbles, when ingested, will cause irregular drop formation and, under certain conditions, may prevent the ink jet head from operating. Finally, known prior art air assisted drop-on-demand ink jet heads are operable at typical maximum drop generation frequencies of approximately 20 kilohertz.
In one prior art attempt to reduce the extent which ink wets the surface surrounding the ink drop-forming orifice outlet, the pressure of the air delivered to the air chamber was increased from a typical pressure of thirty inches of water to fifty inches of water. This increased air pressure did have some affect in reducing the size of the pool of ink which formed at the internal drop-forming orifice outlet. However, some wetting still occurred. Furthermore, this approach increased the velocity at which ink drops were ejected from the external orifice of the ink jet head to the degree that the drops tended to splatter on the print medium. This splattering distorted the resulting image.
Another known approach used to counter the tendency of ink to wet the surface surrounding the internal ink drop-forming orifice outlet is to treat this area with an anti-wetting compound, such as a long chain fluorosilane compound. Such coatings are usually applied as thin coats or even monolayers so as not to greatly alter the characteristics of the internal drop-forming orifice outlet. Such coatings, have been only a temporary solution to the wetting problem. That is, the coatings are frequently sensitive to the constituents of the ink being sprayed, and as such, are soon washed away or contaminated to the extent that they lose their anti-wetting characteristics.
As still another prior art approach directed toward overcoming the anti-wetting problem, European patent application No. 83306260.7 of Soo, owned by Hewlett-Packard Company, discloses the embedding of ions in the surface surrounding an ink drop-forming orifice outlet together with dissolving an oppositely charged ionic anti-wetting agent in the ink. This patent application indicates that this approach reduces the wetting of the surface surrounding the ink drop-forming orifice outlet and facilitates the production of more uniform drops of ink.
Another prior art "Gould" type ink jet head is disclosed in the Dec. 5th, 1983 issue of the "Nikkei Electronics" publication. This type of head utilizes a cylindrical piezo element which expands and contracts in response to driving signals. When the element contracts, an ink chamber or ink surrounded by the element is squeezed to eject a drop of ink from a conical or cylindrical nozzle. Ink passes through a rectifying valve to the piezo element region of the head and a fluid resistance element is placed at the nozzle side of the piezo element region. A larger fluid resistance is provided at the nozzle side of the resistance element than at the rectifying valve to prevent reverse flow of ink at the nozzle side. Also, this article has one figure which appears to disclose a nozzle having a tip inserted partially into an opening through a plate. In addition, air is flowing along the surface of the nozzle and through the opening through the plate.
However, the ink jet head disclosed in the Nikkei Electronics article suffers from a number of drawbacks. That is, the use of valve and resistance elements leads to problems such as manufacturing complexities. The article mentions problems in driving the head above five kilohertz without the rectifying valve. Also, drop frequencies seem to be limited to about ten kilohertz even with the valve. In addition, relatively low air and ink pressures are apparently employed as the air flow is understood to move at approximately the speed of the ejected ink drops rather than to accelerate the generated ink drops. Furthermore, with the nozzle tip inserted into an opening through a plate, the air flow, particularly if increased in velocity, would tend to pull ink from the nozzle tip even without a pulse being applied by the piezo element, thus producing undesired drops.
In addition to air assisted drop-on-demand ink jet heads, non-air assisted ink jet heads have also been utilized, such as exemplified by U.S. Pat. No. 3,747,120 of Stemme. Non-air assisted heads suffer from a number of drawbacks when compared to air assisted heads, primarily in the fact that such non-air assisted heads apply drops of ink to printing medium at limited frequency rates, such as on the order of four kilohertz to six kilohertz.
U.S. Pat. No. 4,312,010 of Doring, U.S. Pat. No. 4,442,082 of Louzil, an article entitled "Ink-Jet Printing" published in 1982 at pages 192-198 of Phillip Technical Review No. 40, by Doring; and an article entitled "Droplet Emission With Micro-Planar Ink Drop Generators", published at page 364 in the SID 1984 Digest, by Doering, Bentin and Radtke; each relate to non-air assisted drop-on-demand ink jet heads with ink nozzles in the form of a projecting circular cylindrical tube with sharp edges and an outer surface in the shape of a ring. This tube projects from an outer wall of an ink chamber and an ink drop-forming orifice outlet is bounded by the inner edge of the ring. Page 194 (FIG. 7) of the "Ink Jet Printing" article, and page 365 of the SID 1984 Digest article, illustrates that wetting is confined to the ring surface. The SID 1984 Digest article also mentions that this surface is wetted symmetrically to provide stable and undeflected droplet emmission. Moreover, the SID 1984 Digest mentions that drop ejection rates of ten kilohertz can be achieved with the illustrated design.
Thus, although these latter references do address the problem of asymetric wetting of the surface surrounding an ink drop-forming orifice outlet, they do so only in connection with a non-air assisted drop-on-demand ink jet head. Moreover, the maximum drop repetition rates are relatively low.
Therefore, a need exists for an improved air assisted drop-on-demand ink jet head which is directed toward overcoming these and other disadvantages of prior art devices.